A like brake system is described in DE-OS 36 03 533. The separating valve is operated hydraulically, a first surface of the valve member being loaded by the pressure in the accumulator, while a second surface of the valve member is acted upon by the pressure in the wheel brake. A weak resetting spring keeps the valve in abutment on a stop, the valve passage being open.
During a braking pressure control action, the pump delivers fluid into the high-pressure accumulator, the pressure of which accumulator moves the valve member in opposition to the effect of the weak spring to bear against a valve seat, whereby the brake line is closed. After a pressure control action, the accumulator pressure is reduced so that the valve member will be reset by the action of the weak spring.
The separating valve serves to accomplish hydraulic isolation of the master cylinder during a control action, thereby preventing the brake pedal from being depressed any further, not even when increased pedal force is applied. Furthermore, no pressure fluctuations due to control may occur in the master brake cylinder which would cause vibrating of the pedal. However, for obtaining such a "quiet" pedal one has to put up with the shortcoming that there is the risk that the separating valve will not switch over after a braking pressure control action and that the brake line remains closed as a result. The consequence of this would be that the vehicle can no longer be slowed down sufficiently. Yet the resetting spring in the brake system according to the cited publication cannot be strengthened to any desired extent, because then it be assured that the separating valve will close during a control action.